1. Field of the Invention
The invention generally relates to integrated optical communications, and more particularly to a slotted multimode interference device for use in telecommunications, sensing, and related applications.
2. Description of the Related Art
Within this application several publications are referenced by Arabic numerals within brackets. Full citations for these publications may be found at the end of the specification immediately preceding the claims. The disclosures of all these publications in their entireties are hereby expressly incorporated by reference into the present application for the purposes of indicating the background of the invention and illustrating the general state of the art.
Multimode interference (MMI) devices are useful for the separation and combination of power of wavelengths or polarizations (SCWP) in integrated optic (IO) systems. However, like all SCWP devices, multimode interference devices can be rather long (geometrically), and thus take up more space on the integrated optical chip than is desirable. For example, wavelength separation/combination devices of 1.3 μm and 1.5 μm are required in the field of fiber optic communications to take advantage of embedded fiber optic systems of sizes of 1.3 μm and to proceed with the deployment of the lower loss 1.5 μm systems. Conventional devices rely on bulk optical filters and therefore suffer high insertion loss. As another example, polarization separation/combination devices are required for various IO and fiber optic sensing systems, such as optical gyroscopes and structural stress sensors. The self-imaging effect is a general property of light, therefore MMI devices do not have to be in IO.
Integrated optic devices are made according to photolithographic and microfabrication techniques. This allows for mass production, in the same way as for electrical integrated circuits. The most common electro-optic substrate materials for integrated optic devices are the semiconductors gallium arsenide and indium phosphide and lithium niobate, a ferroelectric insulating crystal. Lithium niobate is a strong, easily polished nonhydroscopic crystal, with a good electro-optic coefficient. It also has low optical transmission loss.
FIG. 1 illustrates a schematic perspective view of a conventional multimode interference-based SCWP device 1. The device 1 is single mode in the transverse (vertical) direction. The input/output waveguides 2, 3a/3b respectively, are single mode in the lateral (horizontal) direction also. The MMI region 4 supports many lateral modes. The input signal excites these modes, which propagate with different phase velocities down the length of the MMI region, and become de-phased. A self-image of the input 2 to the MMI region 4 forms when the superposition of the modes in the image plane again matches the original modal distribution at the input plane. This condition occurs at planes where the accumulated phase differences among the excited modes are integral multiples of 2π, which allows the excited modes to constructively interfere and reproduce the input's modal distribution. The propagation distance at which this occurs is known as the self-image length, wherein the self-image length depends upon the polarization (either transverse electric, TE, or transverse magnetic, TM) and the wavelength (λ), in addition to other parameters.
A useful property of the self-imaging effect is that a lateral displacement of the input to the MMI region 4 along the object plane results in a corresponding displacement of the self-image along the image plane. This self-image displacement is antisymmetrical for the first self-image length (crossed). The second self-image is antisymmetrical to the first self-image, and thus symmetrically displaced relative to the input (barred). This continues, with odd-numbered self-images being antisymmetrically displaced (crossed), and even-numbered self-images being symmetrically displaced (barred).
In previous patents and publications[1-15], it has been shown that the dimensions of the MMI region 4 can be set such that the modes constructively interfere at the end of the region, forming a self-image of the TE (or λ1) input signal at one output waveguide 3a and TM (or λ2) at the other output waveguide 3b. Thus, the device 1 can separate two polarizations (or wavelengths) from one input 2 into two outputs 3a, 3b, each containing a different polarization (or wavelength). The device 1 can also work in reverse as a combiner. While MMI SCWP devices generally outperform other competing conventional techniques, they are still much longer than necessary or optimal, and some suffer from some performance limitations.
In other prior publications[16-29], it has been shown that introducing changes to the refractive index of portions of the MMI region 4 can drastically alter the self-imaging properties of the MMI device 1, allowing one to switch the light to either of the two outputs 3a, 3b. Most of these conventional devices[16-27] describe devices that require either large changes in the refractive index, or changes to large regions, or both, which limits the usefulness of the respective devices.
Therefore, due to the limitations of the conventional devices and processes, there remains a need for improvements to self-imaging waveguide devices for use in optical and related systems.